This invention is directed to a molding resin composition comprising a poly(biphenyl ether sulfone) resin. More particularly the invention is directed to improved molding resin compositions having reduced tendency toward yellowing comprising poly(biphenyl ether sulfone) resin, and to a method for improving the yellowing resistance of poly(biphenyl ether sulfone) resins.
Poly(aryl ether sulfone) resins have been known for nearly three decades. They are tough linear polymers that possess a number of attractive features such as excellent high temperature resistance, good electrical properties, and very good hydrolytic stability. A variety of poly(aryl ether sulfones) are commercially available, including the polycondensation product of 4,4xe2x80x2-dihydroxydiphenyl sulfone with 4,4xe2x80x2-dichlorodiphenyl sulfone and the polymer of bisphenol-A and 4,4xe2x80x2-dichlorodiphenyl sulfone. These and other poly(aryl ether sulfone) resins are widely disclosed and described in the art, including in U.S. Pat. No. 4,108,837 and Canadian Patent No. 847,963. A third commercial poly(aryl ether sulfone) is the poly(biphenyl ether sulfone) resin available from Amoco Performance Products, Inc. under the trademark of Radel(copyright) R. The latter resin may be described as the product of the polycondensation of biphenol with 4,4xe2x80x2-dichlorodiphenyl sulfone.
Because of their excellent mechanical and thermal properties coupled with outstanding hydrolytic stability, poly(aryl ether sulfones) have found wide application, including in extruded goods and molded articles for use where exposure to severe environments is contemplated. Parts molded from poly(biphenyl ether sulfone) resins have substantially better mechanical properties than those molded from other readily available poly(aryl ether sulfone) resins, and are generally more chemically resistant.
Although poly(aryl ether sulfone) resins are also highly resistant to mineral acids and salt solutions, when exposed to polar organic solvents, they readily stress crack. Where increased environmental stress crack resistance is desired, blends of poly(aryl ether sulfone) resins have been employed to make them more acceptable for use in a solvent environment without substantially affecting their mechanical or electrical properties. Blends comprising polyetherimide resins and poly(aryl ether sulfone) resins are disclosed in U.S. Pat. No. 4,293,670 to have significantly improved environmental stress crack resistance and good impact strength. A variety of other blends have also been described in the art for these purposes including, for example, blends of poly(aryl ether sulfone) resins with polyamide-imides as well as blends with poly(aryl ether ketone) resins. Copolymers with improved stress-cracking resistance are also known in the art including, for example, co-poly(biphenyl ether sulfone) resins comprising bisphenol A moieties as disclosed and described in U.S. Pat. No. 5,164,166.
Poly(biphenyl ether sulfone) resins are known to exhibit good mechanical properties at elevated temperatures and have excellent high temperature stability. These resins may be combined with suitable flame retarding additives to provide highly desirable chemically-resistant, flame retardant, low heat release materials for use in aircraft interiors and the like, as disclosed in U.S. Pat. No. 5,204,400, and are particularly useful for these purposes when employed in blends with polyaryl ether ketones.
One deficiency of formulations based on poly(biphenyl ether sulfone) resins is the tendency of these resins to yellow rapidly when exposed to ultraviolet radiation such as may be encountered when exposed to sunlight or fluorescent lighting. Although the effect on mechanical properties may be minimal, the cosmetic appearance of articles made from such resins may become severely degraded. The color stability of resin formulations, and particularly those supplied in light colors and intended to be used in fabricating articles visible to the consumer, for example, in trim panels and similar appearance parts of aircraft interiors, may be the determining factor in deciding the commercial acceptability of such goods.
This invention is directed to compositions having improved resistance to yellowing comprising a poly(biphenyl ether sulfone) resin, and to a method for improving the yellowing resistance of poly(biphenyl ether sulfone) resins by combining said resin with particular levels of a polyetherimide resin to form a blend. Molded articles comprising such blends have improved resistance toward yellowing and may be further combined with suitable flame retarding additives to provide highly desirable low heat release materials useful in the manufacture of components for aircraft interiors and the like.
Briefly, the poly(biphenyl ether sulfone) resins useful in the practice of this invention comprise the structural unit 
The polymer may further comprise up to 50 mole percent, preferably up to about 25 mole percent, still more preferably no more than about 10 mole percent additional arylene sulfone units with the structure 
wherein Ar is a monoarylene moiety such as p-phenylene, m-phenylene or the like, or is a diarylene moiety other than biphenyl, for example a bisphenol A, bisphenol S or similar moiety. The presence of these additional arylene sulfone units may have a detrimental effect on the overall balance of properties, hence polymers containing only biphenylether sulfone moieties will generally be preferred.
Poly(biphenyl ether sulfone) resins may be prepared by any of the variety of methods well known in the art for the preparation of poly(aryl ether) resins. Two methods, the carbonate method and the alkali metal hydroxide method, are widely known and used for this purpose. In the alkali metal hydroxide method, disclosed for example in U.S. Pat. No. 4,108,837 and in U.S. Pat. No. 4,175,175, a double alkali metal salt of a dihydric phenol is contacted with a dihalobenzenoid compound in the presence of a sulfur containing solvent as herein above defined under substantially anhydrous conditions. The carbonate method, in which at least one dihydric phenol and at least one dihalobenzenoid compound are heated, for example, with sodium carbonate or bicarbonate and a second alkali metal carbonate or bicarbonate is also widely disclosed in the art, for example in U.S. Pat. No. 4,176,222.
The reduced viscosity of the poly(biphenyl ether sulfone) resins useful in the practice of the invention, measured in an appropriate solvent such as methylene chloride, chloroform, N-methylpyrrolidone or the like, will be at least 0.3 dl/g, preferably at least 0.4 dl/g, and the polymer will be film-forming. Generally, resins having a reduced viscosity in the range of from about 0.3.to about 1.0 dl/g will be readily processable thermally and will be useful for these purposes. Very high molecular weight resins, typically those having a reduced viscosity greater than about 1.0 dl/g, may also be found useful, however, such resins will have a high melt viscosity and be difficult to process and will therefore not be preferred.
Polyetherimides suitable for use in this invention are also well known in the art and are described in, for example, U.S. Pat. Nos. 3,847,867, 3,838,097 and 4,107,147. The polyetherimides may be further described as comprising units of the following structure: 
wherein Ar1 and Ar2 are independently selected from substituted and unsubstituted divalent aromatic radicals and more particularly may be monoarylene moieties such as p-phenylene, m-phenylene or the like, or selected from diarylene moieties such as, for example a biphenyl, bisphenol A, bisphenol S or similar moiety. Isomeric structures wherein the ether-linked moiety xe2x80x94Oxe2x80x94Ar1xe2x80x94Oxe2x80x94 may be attached to the 3 and 3xe2x80x2 positions of the respective aromatic rings are also contemplated as within the description of suitable polyetherimides.
These polyetherimides are readily prepared by any of the methods well known in the art such as those disclosed in, for example, U.S. Pat. Nos. 3,833,544, 3,887,588, 4,017,511, 3,965,125 and 4,024,110. Generally, suitable polyetherimides may be made by the reaction of an aromatic bis(ether anhydride) with one or more aromatic diamines selected from m-phenylene diamine, p-phenylene diamine, a diaminodiphenyl ether, a diaminodiphenyl sulfone, a diaminodiphenyl ketone, an aryl-alphatic diamine such as bis-(4-aminophenyl)methane, 2,2-bis-(4-aminophenyl)propane or the like. Also useful in the practice of this invention are the corresponding copolymers wherein up to 50 mole %, preferably no more than 25 mole % of the anhydride units are derived from aromatic dianhydrides such as benzene tetracarboxylic acid dianhydride, benzophenone tetracarboxylic acid dianhydride, diphenylether tetracarboxylic acid dianhydride, napthalene tetracarboxylic acid dianhydride or the like.
Suitable polyetherimides are readily available from commercial sources. Particularly suitable are those containing units derived from 2,2-bis(4-(2,3-dicarboxyphenoxy)phenyl)-propane dianhydride and m-phenylenediamine having the structural formula: 
available from GE Plastics Company under the tradename Ultem(copyright) polyetherimide resin in a variety of grades including Ultem 1010. Also available commercially from the same source are polyetherimide copolymers such as Ultem(copyright) 6000 polyetherimide resin.
Generally, polyetherimides useful in the practice of the invention will have a reduced viscosity greater than 0.2 dl/g, preferably 0.35 to 0.7 deciliters per gram when measured in m-cresol at 25xc2x0 C. Although resins with a reduced viscosity as great as 1.2 dl/g may be found useful, the higher viscosity resins are generally very difficult to process and thus will be less preferred.
The blends of the invention may comprise from about 95 to about 50 wt % poly(biphenyl ether sulfone) and, correspondingly, from about 50 to about 5 wt % of the polyetherimide component. Preferably, the blends will comprise no more than about 40 wt % of the polyetherimide component. Although blends containing higher levels of polyetherimide may exhibit excellent non-yellowing characteristics, polyetherimides are known as generally being brittle resins, and blends containing high levels of these resins possess an inferior balance of mechanical properties. Such blends will therefore not be preferred for most uses.
The compositions of this invention may be prepared by any conventional mixing method commonly employed in the resin compounding art. For example, the poly(aryl ether sulfone) and the polyetherimide may be combined in powder or granular form and fed to an extruder and extruded into strands. The strands may then be chopped to provide pellets for use in a subsequent molding operation.
The invented compositions may be further compounded with other additives including plasticizers; pigments; flame retarding additives, reinforcing fillers other reinforcement such as glass or carbon fibers or the like, thermal-oxidative stabilizers; ultraviolet light stabilizers, processing aids, impact modifiers and the like. Particularly attractive for use in a variety of applications are flame retardant resin formulations comprising the blends of this invention and flame retardants containing zinc borates in combination with fluorocarbon polymers and, optionally, titanium dioxide such as are disclosed in U.S. Pat. No. 5,204,400, the disclosure of which is incorporated by reference.
The invention will be better understood by considering the following examples. These examples are offered to illustrate various embodiments of the invention, but are not intended to in any way limit the scope thereof.